While HIV-1 V3 is known to be important in determining viral cell tropism and coreceptor usage, little is known about the role of SIV V3. The objective of this study is to investigate the significance of SIV V3 for viral cell tropism and coreceptor usage. In this study, we have used three molecularly cloned SIV with distinct cell tropisms, coreceptor usages and the V3 sequences T cell-tropic SIVmacEvT3 using CXCR4 as a coreceptor, human T cell line-tropic SIVmac239 using CCR5, and macrophage-tropic SIVmacEvM3 using CCR5. We have exchanged V3 sequences of each cloned SIV with those of other clones to construct V3 recombinant viruses. Our data obtained from V3 recombinants have shown that the cell tropism and coreceptor usage of each SIV are primarily determined by the V3 sequences. Site-specific mutagenesis studies have further demonstrated that the charge of the amino acids within SIV V3 sequences plays an important role in determining the viral cell tropisms an d corecep tor usage. This suggests that the charge of V3 sequences of SIV, like those of HIV-1, contributes to the formation of coreceptor binding sites in gp120, which may contain the V3 region or be conformationally influenced by it. Interestingly, some of the constructed mutant viruses could use both CXCR4 and CCR5 for virus entry, similar to dual-tropic HIV-1. Since EvT3 using CXCR4, but not 239 using CCR5, induces rapid and severe CD4+ T cell depletion in rhesus macaques, it will be important to examine the pathogenic potential of dual-tropic SIV in vivo. Further studies with the rhesus macaque model using coreceptor-specific SIV clones that utilize CXCR4, CCR5 or CXCR4 and CCR5 will provide important information to better understand the mechanisms and significance of HIV-1 coreceptor changes from CCR5 to CXCR4 in AIDS patients. FUNDING NIH RR00163 (Project 2) PUBLICATIONS Martin K, Hagen S, Kodama T. Genetic determinants of SIV CXCR4 usage the role of V1 and V3 sequences. In 16th Annual Symposium on Nonhuman Primate Models for AIDS (held in Atlanta, GA, October 7-10, 1998) (abstract 3).